Recovery of natural gas liquids such as ethane, propylene, propane, butylene, butane, and heavier components from refinery fuel gas streams is of economic interest due to the incremental value of the liquid products over the value of the fuel gas. Propylene, butylene, butane, and the heavier components are currently of particular interest due to their having a higher incremental value than ethane or propane.
The presence of carbon dioxide in the fuel gas stream plays a significant role in the percentage of NGL products that are economically recoverable. Generally, the more carbon dioxide in the fuel stream, the more attention that must be paid to both its concentration level and its temperature in order to avoid freezing this carbon dioxide. In many cases involving fuel streams not having a high carbon dioxide concentration, higher recovery is often achievable by lowering the temperature of the process. This however cannot be so easily accomplished with significant amounts of CO.sub.2 in the fuel stream due to solid carbon dioxide formation. Removal of the CO.sub.2 upstream of the NGL recovery unit may be done with amines (DEA or MEA). This would eliminate the problem of solid CO.sub.2 formation in the cold sections of the NGL recovery unit but it would significantly add to the installation and operating cost of the process.
In addition to carbon dioxide, there is often a high molar concentration (30% to 60%) of hydrogen in the refinery fuel gas stream. This hydrogen acts as a noncondensible inert at the normal temperatures and pressures encountered in a typical NGL recovery unit. Consequently, this high molar concentration of hydrogen necessitates higher pressures (800 psi) and lower temperatures (-160.degree. F.) than are required for comparable NGL recovery rates utilizing an inlet gas in which methane is the most volatile component. The presence of CO.sub.2 in the hydrogen rich stream serves to limit NGL recovery percentages to even lower levels than would be expected for a methane rich stream.
Another factor which limits economical NGL recovery percentages is the incremental value of the NGL components over that of the fuel. Currently, ethane and propane have low incremental value while propylene, butylene, butane, and the heavier components have a relatively higher incremental value. The ideal process then would reject the low value ethane and propane and recover the high value components. Recovery of the high value propylene, however, forces incidental recovery of the lower value propane because propylene is more volatile than propane. Rejection of the low value ethane in a distillation tower withut a controlled reflux system is impossible without also suffering a partial rejection of the high value propylene. Although rejection of the ethane is feasible in a standard turbo-expander plant, the high hydrogen concentration of the stream forces very low operating temperatures. These lower temperatures are necessary to compensate for the propylene rejection which will occur in the unrefluxed turbo-expander plant de-ethanizer.
A classical reflux system on the de-ethanizer overhead is also not economical due to the low operating temperature level required. The cost of a refrigeration system to provide refrigeration at the required temperature level (approximately -160.degree. F.) would be prohibitive. In addition, if CO.sub.2 is present in the process, solid formation at this temperature level may occur, thereby disrupting operation. Several schemes have been proposed which provide a liquid feed to the top of the cryogenic column. These schemes do allow slightly warmer temperatures for comparable recoveries, but are of limited use because the process schemes are not true reflux systems. Furthermore, the flowrate of the liquid feed to the top of the column or the temperature of the stream or both are limited by other process constraints.
Another system that is known is described in U.S. Pat. No. 4,507,133 and also in the article entitled Expander-Gas Processing Plant Converted, Oil & Gas Journal, June 3, 1985, written by Schuaib A. Khan with Esso Resources Canada Ltd, Calgary. This system, however, addresses methane-rich gas streams which are wholly lacking in any hydrogen or carbon dioxide concentration. It is exactly the complications arising from the inclusion of hydrogen and carbon dioxide in the fuel supply stream that the present process addresses.
Consequently it is an object of this invention to recover a high percentage of propylene and heavier components without rejection of incidentally recovered ethane and lighter components and to do so with a standard turbo-expander plant without closely approaching the temperature at which solid CO.sub.2 is formed. The proposed process uses this method to produce a raw NGL stream with a high percentage recovery of propylene and heavier components. One unique feature of the present process involves sending the raw product to a second distillation unit where ethane and lighter components are rejected. Only a small amount of methane and hydrogen are present in the overhead of the second column. This allows a classical reflux system to be employed with modest refrigeration temperature levels. The rejected ethane from the second column overhead may be mixed with the residue gas from the first column, or it may be condensed and subcooled and used as a top feed to the first column to further enhance recovery levels.
It is another object of this invention to extract natural gas liquids from fuel gas streams that have a high inert (hydrogen) content and a high carbon dioxide content and do so under lower pressures than heretofor been possible and with higher temperatures so as to eliminate the problem of solidifying CO.sub.2.